First Observation of the Ground-State Hyperfine-Structure Resonance of the Muonic Helium Atom

The first measurement of the hfs interval $\Delta \nu$ for the muonic helium atom ${\left({}_{}{}^4{}_{}{}^{}\mathrm{He}_{}^{++}{}_{}{}^{}{\mu }^{-}{e}^{-}\right)}^0$ is reported. In terms of its electronic structure, it is a heavy isotope of hydrogen. Polarized atoms are formed by stopping polarized negative muons in a helium-gas target at 19.4 atm with a 1.5% admixture of Xe. The ground-state hfs splitting $\Delta \nu$ was measured through observation of a microwave magnetic-resonance transition at zero magnetic field. After correction for the hfs pressure shift, we determine $\Delta \nu =4464.95\mathrm{}\left(6\right)\mathrm{}$ MHz (13 ppm).